The invention relates to an apparatus for crystal pulling, preferably from the melt, especially a monocrystal pulling apparatus, with a lock valve which is disposed between a pulling pot and a lock chamber. The valve is opened at the start of the pulling process thereby releasing a lock entrance for the crystal to be pulled. A closure together with a gasket closes the lock entrance after the pulling process has ended and thus hermetically separates the pulling pot from the lock chamber.
In the field of crystal growing a great number of different processes are known, such as crystal growing from the gaseous phase, from a solution, and from the melt. The various methods for growing crystals from the melt have achieved a leading position over other growing methods on account of its highly developed process technology and productivity.
Crucible processes of many different kinds are used in growing crystals from the melt. There is, for example, the so-called Kyropoulos process which is characterized by dipping a chilled seed crystal into the melt. Furthermore there is the so-called Czochralski process in which a crystal is pulled from the melt.
There is also the Bridgman process, which is characterized by a vertical lowering of a crucible in the temperature gradient. Lastly, there is vertical zone melting without crucibles.
The lock valve of the present invention is generally usable wherever it is important to protect one or more gaskets on the closure of a lock valve against the thermal radiation of the hot crystal during the crystal pulling process.
In Leybold's publication 45-100.02 entitled, "Crystal Growing," apparatus and methods are described for growing crystals from the melt.
The state of the art also includes the laid-open European patent application 0 389 2784, which describes a lock valve used in a monocrystal pulling apparatus between a main chamber and an upper pull chamber. The lock valve has a frame-like body, and a vertical connecting passage between the main chamber and the upper pull chamber.
The closure of the lock valve can be rotated about an axis and moved up and down by a shaft. There is a lever for turning the shaft on its axis. A drive means is furthermore provided which can move the shaft up and down. A horizontal arm mounted on the shaft bears the closure and holds it in the horizontal position. A flange surrounds an opening which forms the connecting passage between the main chamber and the upper pulling chamber.
The European patent application discloses an opening in the circular closure, by which a circular cylinder is formed. The flange described above has a circular margin that is directed vertically upward. The outside diameter of this margin is slightly smaller than the inside diameter of the circular cylinder. The circular cylinder of the closure, when the lock entrance is closed, is slipped, so to speak, over the circular margin of the flange. An annular gasket is disposed in a groove made in the outside wall of the margin of the flange. When the circular cylinder of the closure is slipped onto the closure this gasket comes in sliding contact with the inside wall of the circular cylinder of the closure.
Sliding gaskets require carefully prepared surfaces. In spite of the careful preparation of the friction surfaces, sliding gaskets have but a very limited life. This is especially true if, as in the present case, sliding gaskets are used in apparatus for pulling crystals from the melt, no grease or oil lubrication of the gasket may be practiced. For technical reasons the working atmosphere of the crystal pulling apparatus must remain free of grease and oil. The limited life of the gasket according to the European disclosure is therefore explained substantially by the severe shearing stress applied to the gasket.